1. Field of the Invention
This invention relates to a process for solvent dewaxing waxy hydrocarbon oils. More particularly, this invention relates to an improved process for dilution chilling dewaxing waxy petroleum oil stocks in a staged chilling zone wherein cold dewaxing solvent is injected into said zone at a plurality of stages therealong and wherein the cold dewaxing solvent and the waxy oil are substantially instantaneously mixed in each stage as the waxy oil-solvent mixture passes from stage to stage, the improvement comprising modifying the temperature profile along said zone by adjusting the cold solvent distribution to each stage so that the temperature drop is greatest in the first stage into which cold solvent is injected with the stage to stage temperature drop progressively decreasing as the waxy oil-solvent mixture progresses through said chilling zone. This invention is particularly useful for dewaxing waxy lubricating oil stocks.
2. Description of the Prior Art
It is well known that wax-containing petroleum oil stocks can be dewaxed by shock chilling with a cold solvent. It is also known that shock chilling, in itself, results in a low filtration rate of the dewaxed oil from the resultant wax/oil-solvent slurry. It is now well known that the harmful effects of shock chilling can be overcome by introducing the waxy oil into a staged chilling zone and passing the waxy oil from stage to stage of the zone, while at the same time injecting cold dewaxing solvent into a plurality of the stages and wherein a high degree of agitation is maintained in the stages so as to effect substantially instantaneous mixing of the waxy oil and solvent. As the waxy oil passes from stage to stage of the cooling zone it is cooled to a temperature sufficiently low to precipitate wax therefrom without incurring the harmful effects of shock chilling. This technique produces a wax/oil-solvent slurry wherein the wax particles have a unique crystal structure which provides superior filtering characteristics such as high filter rates and high dewaxed oil yields. The basic concept of dilution chilling dewaxing is disclosed in U.S. Pat. No. 3,773,650, the disclosures of which are incorporated herein by reference and will hereinafter be referred to as DILCHILL for the sake of brevity.
A number of improvements and modifications have been made to the basic concept of DILCHILL. U.S. Pat. No. 3,642,609 shows that in a vertically staged cooling tower, the velocity of the solvent at the injection points within each stage should be at least 5-30 times that of the peripheral velocity of the mixer blades. This results in greater filtration rates and higher dewaxed oil yields than could otherwise be obtained without the relatively high velocity solvent injection. In U.S. Pat. No. 3,775,288 is disclosed a combination of dilution chilling with scraped surface chilling for dewaxing lubricating oils. U.S. Pat. No. 3,681,230 discloses adjusting the dewaxing solvent composition so that the waxy oil and solvent are immiscible near the last stage of the cooling zone. This results in superior dewaxed oil yields and higher filter rates when the waxy oil stock being fed to the tower is relatively high in viscosity and molecular weight. U.S. Pat. No. 3,850,740 discloses partially prediluting the waxy oil when same is a relatively heavy feed such as a resid or a bright stock, before the oil is introduced into the chilling zone.
However, in all of these DILCHILL dewaxing processes it was thought that the rate of solvent addition to each stage should be adjusted so as to obtain the same or approximately equal temperature drops in each stage. For example, U.S. Pat. No. 3,773,650 in column 6, lines 7-11, discloses adding cold solvent so as to give equivalent temperature drops per stage. Similarly, in U.S. Pat. No. 3,775,288 at column 4, lines 38-43, it is disclosed that the same temperature drop should be maintained from stage to stage of the chilling zone. Further, all of the DILCHILL dewaxing processes in commercial use up to the present time have been designed for and operated with equal temperature drops per stage. It was thought that this was the optimum temperature profile and method of solvent distribution, since it is well known to those skilled in the art that the shock chilling inherent in a large, sudden temperature drop, particularly in the early stages of wax precipitation, tends to cause excessive nucleation, the production of many fine crystals, and hence, poor filtration and relatively high liquid to solids ratios in the wax cake. Therefore, it was relatively unexpected to discover that modifying the temperature profile in a DILCHILL dilution chilling zone so as to provide the greatest stage to stage temperature drop in the early stages in which wax precipitation occurs would result in an improvement in the process as measured by higher dewaxed oil filter rates and lower liquid to solids ratios in the wax cake.